This chapter examines the definitions and meanings of happiness, well-being, and eudaimonia, seeking a plausible way forward. It describes the properties of subjective well-being (SWB) and discusses several possible advantages of using it as a primary definition and measure of well-being. There is a fear that SWB misses something important—intuition tells us that "there must be more to a good life than just positive mood and satisfaction." The chapter presents the Eudaimonic Activity Model (EAM), which takes advantage of these properties to provide a general model for testing eudaimonic theories. It considers further problems in the EWB literature, and shows how the EAM handles or resolves these problems. The chapter shows that researchers' failure to make the distinction has contributed to erosion in scientific precision, and lost opportunities for understanding how positive change actually occurs. It suggests that the EAM supplies a potentially valuable framework for testing and comparing different eudaimonic theories and constructs.